DE8426208U1 - Cover for the nozzles of a vacuum ink print head - Google Patents

Description

AGFA-GEVAERT Public limited company D-5090 Leverkusen Patent Department

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Cover for the nozzles of a vacuum ink print head

The invention relates to a cover for the nozzles of a vacuum ink print head.

The use of quick-drying inks, e.g. solvent inks, in inkjet print heads using the vacuum ink injection process requires aids that prevent the ink from drying out at the nozzle ends when the print head is idle or not in use.

The ink menisci that form in the nozzles must not be touched, nor must ink be pushed back into the nozzle channels by external pressure. The pressure that forms in front of the nozzles must therefore be smaller than the surface tension of the ink menisci in the nozzle ends.

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The invention is therefore based on the object of creating a closure device for an inkjet print head which seals the nozzles airtight and therefore protects them from drying out without exerting a closing pressure on the ink menisci in the nozzles of the print head.

This object is achieved according to the invention by the features of the main claim. Further advantages and features of the invention can be found in the subclaims.

The invention is explained in more detail with reference to drawings.

Fig.l a section through a cover according to the invention for a vacuum ink print head,

Fig.2 a section through another embodiment

a cover according to the invention, 20

Fig.3 shows a further embodiment of a cover according to the invention and an arrangement for bringing together the print head and cover.

All images shown are greatly enlarged. To better understand the invention, the functional principle of a vacuum ink print head is briefly explained using Fig. 1. The print head as a whole is designated 1, its ink spray nozzles 2, its front side with the outlet openings of the nozzles 2 1a, the ink reservoir 3 and ink supply lines that lead from the reservoir 3 to the nozzles 2

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lead, designated 4. A little in front of the outlet opening of each nozzle 2, a piezoelectric tube 5 is arranged in each nozzle 2. If voltage is applied to it, it contracts and presses a droplet of ink to the outlet opening of the nozzle onto a sheet of paper lying in front of it that is to be printed. If the voltage is switched off again by the piezoelectric tube 5, it expands again. The ink surface in the nozzle outlet opening forms a meniscus that is concave towards the outside, which is very faintly indicated and visible in Fig. 1 and has a clear surface tension. This is so great that when the piezoelectric tube 5 expands, the ink cannot be sucked back from the nozzle outlet opening into the nozzle 2, but rather new ink is sucked in from the reservoir 3. Especially with quick-drying inks, which therefore contain solvents that evaporate quickly, there is a risk that these ink menisci will dry out in the nozzles if the print head 1 is not in use for a long time and is not in use. On the other hand, there is a risk that when a cover is put on that closes off the nozzles 2 with the ink menisci, the current air pressure will be so high that the surface tension of the ink menisci in the nozzles will not hold up and these menisci will be destroyed and the ink can then flow from the nozzles back into the piezoelectric tubes 5 and the print head will therefore become inoperable. Therefore, the present invention creates a special cover for such an ink print head 1, which is designed in such a way that when such a cover is put on

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cover 6 onto the front side la of the print head 1, the meniscus-shaped ink surfaces in the nozzles 2 are preserved and cannot dry out.

A cover 6 according to the invention consists of a frame or support part 7 which, for example according to Fig. 1, has a circumferential groove 7a on its cover face facing the print head face la, into which a sealing ring 8 is inserted, e.g. glued. At a short distance from the cover face and thus from the sealing ring 8, a very soft membrane 9 is inserted, e.g. clamped or glued, into the frame 7. Its inherent tension is lower than the surface tension of the ink menisci in the nozzles 2. The membrane 9 seals the frame 7 airtight to the outside. If the cover 6 with its sealing ring 8 is pressed against the face la when the print head 1 is not in use, the space 10 between the outlet opening of the nozzles 2 and the membrane 9 is sealed airtight; the sealing element 8 forms a pressure chamber 10 together with the membrane 9. If the pressure, e.g. due to the lid 6 being moved too suddenly, becomes greater than the surface tension of the ink menisci, the membrane 9 immediately and first of all moves outwards so that the pressure cannot affect the ink menisci at all. The pressure equalization in the pressure chamber 10 caused by the membrane 9 prevents the ink menisci from bursting and drying out despite the airtight seal of the nozzles 2. The pressure chamber 10 shown greatly enlarged in the figures is in reality very small. This causes a rapid

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Saturation of the air in the printing chamber with moisture As a result of the ink solvent initially evaporating, so that drying out of the nozzles 2 and the ink surfaces is safely avoided even over a long period of time.

In Fig. 2, another embodiment of a cover 6 according to the invention is shown. In all figures, parts that function in the same way are provided with the same reference numbers. According to Fig. 2, the membrane 9 is designed like a hood and has an edge bead 8 on its open side facing the front side la, which forms the seal. The membrane 9 and seal 8 are therefore designed as one piece, with the edge bead 8 being connected to the frame 7 and protruding slightly above it.

According to Fig.3, the actual closure part of the cover 6 is designed as a completely hollow suction cup 8 made of soft rubber or plastic, which is supported on the back by a frame part 7. The membrane 9 is placed on the open back of this tubular suction cup 8, which is also designed like a suction cup, and is pressed airtight by a clamp-like spring frame 11. The membrane 9 is thus sucked on by the back of the suction cup, and when the front of the suction cup is pressed against the front face of the print head 1a, the former also sucks itself onto the latter, so that the described airtight pressure chamber 10 is created.

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As the various embodiments show, it makes sense from a manufacturing point of view to manufacture at least some of the lid parts in one piece. However, it is also possible in principle to manufacture an entire lid according to the invention in one piece from elastomers with different shore hardnesses. The plastics used must of course be chemically resistant to the ink materials and solvents.
10

Now it would in principle be possible to mount the print head 1 so that it can be moved in the direction of the spraying so that when not in use it is pushed back from the plane of the sheets to be printed, and then to provide the cover 6 with a sleeve and to put it over the front side 1a of the print head 1 from the front so that it stays there. All other usable, known fastenings for the cover could also be used. However , Fig. 3 shows a particularly advantageous arrangement for bringing the cover 6 and the print head 1 into contact. Here the print head 1 is mounted so that it can be moved in a straight guide 13 of a rotatable disk 14. In the position shown in solid lines the print head 1 is turned and pushed against the suction cup 8 and is therefore hermetically sealed by the cover 6, 8 in the manner described. If the print head 1 is to be brought into its printing position, it is first moved in the direction of arrow 15 away from the cover 6 in the guide 13 and then rotated 90° clockwise by means of the disk 14 and moved in the direction of arrow 16 against the surface 17 to be printed into its functional position. This

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The functional position is shown in dash-dotted lines. If the print head is to be hermetically sealed again, it is moved back in the opposite direction. This turning and sliding movement can of course be motorized and triggered by a simple control button or keyboard.

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Claims (10)

ilt llf tt · ft«· &igr;&igr; ■ &igr; · &igr; 4 &igr;&mgr; · &igr; »«j» * &igr; jo · Claims 1. Cover for the nozzles of a vacuum ink print head, characterized in that it has a circumferential sealing element (8) which, when fitted, rests against the front side (la) of the print head (1) and a membrane (9) arranged in the flat side of the cover, and that the sealing element (8) and the membrane (9) together form a pressure chamber (10). 2. Cover according to claim 1, characterized in that the inherent tension of the membrane (9) is smaller than the surface tension of the meniscus-shaped ink surfaces that form in the nozzle ends of the print head (1) when not in use. 3. Lid according to one of the preceding claims, characterized in that the membrane (9) and the sealing element (3) are fastened in or on a lid frame (7). 4. Lid according to one of the preceding claims, characterized in that the seal (8) and the membrane (9) are made in one piece from plastic and this one-piece membrane-seal part (9, 8) forms the pressure chamber (10). A-G 1990 Ii »■' • I · · 5. Lid according to claims 3 and 4, characterized in that the lid frame (7) has a cylindrical opening in which the one-piece membrane sealing part (9, 8) lies, and that it is fastened, preferably molded, with its sealing edge (8) to the pressure head-side edge of the lid frame (7). 6. Cover according to one of claims 1 to 3, characterized in that the membrane (9) is clamped into the cover frame (7) and that the sealing element (8) is inserted as a sealing ring into a corresponding groove (7a) on the print head-side inner edge of the cover frame (7), preferably glued in. 7. Lid according to one of the preceding claims, characterized in that it is formed by an elastomer molding, the elements of which consist of elastomers of different Shore hardnesses. 8. Lid according to one of the preceding claims, characterized in that the sealing element (8) is designed as a tubular suction cup, one side of which can be pressed onto the pressure head (1, 1a) and onto the other side of which the membrane (9) is pressed by means of a frame-shaped spring (11). A-G 1990 - &iacgr;&ogr; - 9. Cover and ink print head according to one of the preceding claims, characterized in that the ink print head is mounted so as to be movable relative to the cover (6), that it can assume a position for printing on a sheet (17) to be printed and a further position for closing it with the cover (6), and that the cover (6) and print head (1) are mounted so as to be movable relative to one another in the second position for closing the latter or away from one another for opening. 10. Cover and ink print head according to claim 9, characterized in that the print head (1) is mounted radially displaceably in a rotatable disk (14) and can be rotated into a position opposite the sheet (17) to be printed and into a position offset at an angle thereto, opposite the cover (7, 8, 9), and can be pushed onto the sheet (17) or against the cover. 2 sheets of drawings A-G 1990